Great question — answering as the UphillTech team:
Velcro with an adhesive backing may do the trick. You just stick one "dot" on the tail of the watchband, and the other on the watch band so that it aligns with the first dot when you're wearing your watch. Be careful when putting on and taking off your watch not to snag the Velcro dot that is on the tail. It may come off over time.
Full answer by BrettFromLA: https://lifehacks.stackexchange.com/a/8016 (CC BY-SA 3.0).
Need it done for your business? UphillTech.